target/openrisc: Optimize for r0 being zero

The HW does not special-case r0, but the ABI specifies that r0 should
contain 0.  If we expose this fact to the optimizer, we can simplify
a lot of the generated code.  We must of course verify that r0==0, but
that is trivial to do with a TB flag.

Signed-off-by: Richard Henderson <rth@twiddle.net>